The present invention relates to the optimization of dissimilar chillers in a building air conditioning system in order to meet building load conditions with a minimum expenditure of energy.
The air conditioning systems of large buildings typically comprise a plurality of chillers which are connected both to cooling coils located within fan system ducts and one or more cooling towers located outside of the building. The fan system will typically comprise a fan located in a duct which draws air from an outdoor air duct and from a return air duct, passes this air over various heat exchange coils and, perhaps humidifiers, and supplies this treated air to the zones connected to the fan system. The heat exchange coils may include both cooling and heating coils and the ducts connecting this source of treated air to the individual offices or spaces to be supplied by the fan system may include dampers and/or reheat coils for controlling the amount of treated air supplied to the zone and for further treatment of the air. Chilled water is then circulated between the chiller and the cooling coil in this fan system for cooling the air moving through the duct system. The chiller is also connected to a cooling tower where a separate supply of water is circulated between the chiller and the outside of the building. Freon is typically circulated within the chiller for transferring the heat content of the water circulated to the cooling coil to the water circulating between the chiller and the cooling tower. Thus, the cooling tower then delivers this heat to the outside of the building.
Large chiller plants within such buildings are composed of multiple chillers, pumps and cooling towers. The total chiller plant is sized to supply maximum load. For intermediate loads, the choice of the proper combination of chillers to meet load conditions can have a significant impact on total plant efficiency. In order to improve total plant efficiency, the selection of the proper combination of chillers to meet load conditions must be optimized. U.S. Pat. No. 4,210,957 discloses an optimization system for a plural chiller plant where the chillers all have similar efficiency characteristics. The present invention is directed to those chiller plants which are comprised of chillers having dissimilar efficiency characteristics. In the present invention, the part load characteristics of chillers and how they interact in meeting a given load are the essential variables in the optimized decision making process.
The electrical energy input necessary to produce a given amount of cooling is the primary variable. The electrical energy input will vary not only with cooling load but also with refrigerant head. Refrigerant head is the pressure change which the refrigerant compressor must produce. Thus, the energy required by the compressor for a given load is directly related to the magnitude of the refrigerant head. The refrigerant head is the difference between the condensor pressure and the evaporator pressure. Since condensor pressure is related to the inlet condensor water temperature and evaporator pressure is related to the outlet chilled water temperature, the refrigerant head can be indicated by condensor supply temperature minus chiller supply temperature. And the rate of heat removal from the building chilled water system is the cooling load on the chiller plant. Cooling load is normally represented in tons which are 12,000 BTU per hour. The normal measurement of this load is at each chiller by measuring chilled water flow rate and temperature drop across the chiller using the formula BTU load=flow (GPM).times.(temperature in-temperature out). Tons of load is then the BTU load divided by 12,000.
The chiller plant of larger buildings, or complexes of buildings, is usually the largest single user of energy in the facility. It provides the chilled water used by fan systems to air condition spaces. Chiller plants not only use the most energy supplied to the building but are the greatest influence on electrical demand charges. Chiller plants typically have multiple chillers and associated pumps and cooling towers. How and when this equipment is used can make a significant difference in the overall chiller plan efficiency. Specific plants will have variations in chiller type, size and system arrangement that will influence optimum control strategy. Moreover, actual operating environment and equipment condition are major influences on the actual part load energy efficiency characteristics of chillers, meaning that the efficiency characteristics of chillers will change depending upon operating environment and equipment usage. Thus, not only must the chillers be selected for maximum efficiency based upon refrigerant head and building load, but the efficiency characteristics stored in the system must be periodically updated to reflect actual conditions. While the function of a chiller plant is to provide the amount of cooling required by the air conditioning load, the function of the chiller plant optimization control system is to determine the best plant mode of operation for changing loads. The multiple chiller selection using the appropriate part load characteristics is an analysis that determines the best combination of chillers to meet existing or anticipated loads. Although the present invention can be used with any chiller plant that uses more than one chiller, it is particularly useful for chiller plants having dissimilar chillers and/or chillers having dissimilar efficiency characteristics.